The present invention relates to a zoom lens system adapted for use in a compact camera and which is subject to less strict conditions on back focus than a lens system used in a single lens reflex camera.
The following three types of zoom lens systems are available for use in compact cameras.
(I) A telephoto two-group type composed of a first lens group having a positive focal length inclusive of a stop diaphragm and a second lens group having a negative focal length. An example of this type of lens system is disclosed Japanese Unexamined Published patent applications Nos. 128911/1981, 201213/1982, 48009/1985 and 170816/1985.
(II) A three group type which is a modification of type (I) and which is composed of a first lens group having a positive focal length inclusive of a stop diaphragm, a second lens group having a positive focal length, and a third lens group having a negative focal length. An example of this lens type is disclosed Japanese Unexamined Published patent application No. 184916/1983.
(III) A four-group type composed of a first lens group having a positive focal length, a second lens group having a negative focal length, a third lens group having a positive focal length, and a fourth lens group having a negative focal length. An example of this lens type is disclosed in Japanese Unexamined Published patent application No. 57814/1985.
These zoom lens systems, however, have the following problems:
The two-group type (I) lens system has a very simple construction, but it requires the first and second lens groups to be displaced by a large amount in order for zooming.
The three-group type (II) lens system only differs from type (I) in that the first lens group is divided into two positive lens units, and hence it also requires a substantial amount of lens displacement for zooming.
The four group type (III) lens system requires a smaller amount of lens displacement than the first two types. However, the use of four lens groups renders the lens configuration very complex. In addition, the individual lens groups, in particular, the second and third groups have such large powers that any slight error introduced in the manufacturing process will cause serious effects on the performance of the resulting lens system. Therefore, this type of lens system is rather difficult to manufacture.
Moreover, all three prior art of zoom lens system types (I), (II), (III) have one common problem: since they are intended to be used in compact cameras, they have no conditions of constraint at all on the back focus. Some have a back focus as short as about 1 mm. In the absence of any constraints on the back focus, a zoom lens system having a reduced overall length can be easily constructed. On the other hand, the diameter of the lenses in the rear group is increased and the dimensions of the camera body, including various mechanisms such as zooming and auto-focusing units, cannot be substantially reduced. A further problem is that internal reflection is highly likely to occur between the film surface and the lens surface in the rear group.